Local failure remains a serious problem in cancer treatment. Of the more than 1 million cancer patients who will present for treatment this year, about 1/3 will fail their treatment due to local recurrence alone, or local recurrence with distant metastasis. In radiation therapy, major efforts are being made to implement 3-dimensional (3D) conformal treatment techniques to improve local control. Accurate treatment setup becomes vital, spurring the development of on-line electronic portal imaging devices (EPIDs) to enhance routine treatment verification. Unfortunately, the inherently poor image quality and high imaging dose associated with megavoltage (MV) imaging render current EPIDs ineffective for conformal treatment verification. It is our hypothesis that adequate assessment of treatment errors cannot be made with on-line MV imaging. An image similar in quality to the prescription kilovoltage (kV) image is essential. A kV source on the treatment machine will allow us to make low dose, diagnostic quality images and greatly improve the detection and correction of setup error. In the 4 year research period, our specific aims are to: (1) design, implement and optimize an on-line dual beam imaging system for small field conformal treatment verification, (2) test the hypothesis by determining, with phantom imaging studies, the improvement in setup error detection using dual beam imaging as compare to conventional MV imaging, and (3) develop approaches to improve setup error correction using the additional 3-dimensional (3D) information in multiple projection kV images. A kV x-ray source and two on-line imagers will be mounted on an accelerator. A diagnostic quality dual beam image is produced by acquiring a kV open field image of the patient in treatment position and superimposing it with the MV portal image. The ability of a group of observers to detect setup variations in the dual beam and the MV images will be evaluated. As more projection images can be taken with the low dose kV beam, linear transformation methods will be applied to the images to more accurately solve for the cause of 3D setup error. Finally, the availability of a kV beam also enables us to implement CT scanning capability on the accelerator to study soft tissue changes during the time of treatment. Our findings will improve our knowledge about the limit of setup accuracy in radiation therapy and provide important information for the development of real-time setup error correction strategies in the future.